[Public WebGL] WebGL back buffer contents

Tim Johansson [email protected]
Thu Jan 28 10:39:13 PST 2010

On 2010-01-28 18:27, Chris Marrin wrote:
> On Jan 28, 2010, at 1:43 AM, Tim Johansson wrote:
>>> In recent conversations with people at NVIDIA they were pretty adamant
>>> that we not mess with the OpenGL state, in particular the viewport,
>>> upon changing the canvas size. Gregg's render target example is a good
>>> reason to not automatically call glViewport() behind the scenes.
>>> Another point is that it isn't obvious exactly when the implicit call
>>> to  glViewport() would be made -- whether it would be synchronous with
>>> the setting of the width / height properties or whether it would
>>> somehow be done lazily, before the first draw call after the canvas is
>>> resized.
>>> I used to think that doing this automatically would be a good idea but
>>> because of the semantic details I don't any more.
>> We recently said that resizing should clear the canvas, which means 
>> we have to mess with the gl sate anyway. We will at least need to 
>> change bound FBO to 0 and disable scissor testing. Does it not 
>> qualify as messing with the state if we mess with it and then try to 
>> restore it?
> Why would we have to change the FBO or scissor state? If you have an 
> FBO set, then changing the size of the drawing buffer doesn't affect 
> it, does it?
When resizing the canvas what you have to do is first resize the 
backbuffer which is probably an FBO or pbuffer (unless you created a 
bigger backbuffer than you needed to avoid having to resize it).

After that you need to clear the backbuffer, otherwise resizing and 
drawing a small triangle could give undefined results. Assuming you want 
to call glClear to clear the backbuffer you have to switch FBO or you 
will clear the applications FBO rather than the backbuffer.

If the application has a scissor set and you clear without disabling 
scissor test you will only clear the area defined by that scissor rect, 
which might not be the entire new backbuffer.

Changing the size of the drawing buffer will not affect the FBO, but it 
will affect the drawing buffer in a way that is difficult to do without 
changing the GL state.

>> I agree that in the case of rendering to an FBO you would not want to 
>> change the viewport automatically, but I would rather have apps 
>> handle that case themselves and set the viewport back than have apps 
>> handle the most common case themselves. If the spec says that 
>> changing width or height behaves as if it calls viewport as soon as 
>> the width/height attribute is changed it would be clear what will 
>> happen. I would not even call it messing with the state as it just 
>> does what the app tells it when the app tells it to, that is change 
>> the size of the buffer and the viewport.
>> We could also do something more magical and say that if you have not 
>> changed the viewport yourself it will automatically change. If you 
>> ever did call viewport automatic viewport adjustment is disabled. 
>> That way it would never change the viewport for apps actually 
>> modifying the viewport and it would just work for apps who does not 
>> care about the viewport. So initial value for viewport would be 
>> "automatic", but there is no way to set it back to "automatic" if you 
>> change it yourself.
> I'm not sure of your concern. What is the problem with requiring that 
> the author calls ctx.viewport() after changing the width and height? 
> The author has to know about viewports and keeping track of them. So 
> why is that an issue?
If you have to call viewport every time you resize the canvas then yes, 
you have to know about it. If it changes automatically you would not 
have to know about viewports if you just want to render to the full 
canvas and don't want to change the viewport.

It is not a big issue, just seems like an inconvenience that can be fixed.


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